Experiments with ultraviolet and infrared light reinforce the idea that electromagnetic radiation extends beyond the visible wavelength range. Experiments with ultraviolet light, in particular, are visually impressive. For advanced students, these experiments can show how light reveals the structure of atoms.
The best UV light sources produce both long-wavelength (300-400 nanometer "black light") and short-wavelength (less than 300 nanometer) light. Fluorescent minerals or dyes, which absorb the UV light and emit it as visible light, create a spectacular demonstration. If you switch the UV light from long-wavelength to short-wavelength, you will see a difference in the color (wavelength) of the emitted light. The phenomenon of fluorescence involves the structure of atoms (see Inside an Atom).
The ASP's Project ASTRO activities handbook, The Universe at Your Fingertips, describes an experiment to test whether there is light below the red edge of the visible range. This experiment involves three thermometers, which measure the temperature of the air where the experiment is being done. Break sunlight into a spectrum using a prism and place the thermometers at three points in the spectrum: one in the violet range, one in the yellow range, and one just barely beyond the red end. What do the thermometers read?
Another source of confusion is the different results students get when mixing colors of light, as opposed to mixing colors of pigments. Mixing red, green, and blue light makes white light because these are the wavelengths that comprise white light. By contrast, mixing red, green, and blue paints creates black the absence of light and color. This happens because the red paint absorbs all colors except red light; the green and blue pigments absorb the red. Only when we confront our misconceptions can we can begin to replace them with facts.
DEBRA FISCHER is a graduate student in astronomy at the University of California in Santa Cruz. As if analyzing star formation weren't enough, Fischer volunteers for the ASP's Project ASTRO, coordinates the science fair at Commodore Sloat Elementary School in San Francisco, and runs Lick Observatory's "Ask an Astronomer" program. Oh, and she's the mother of three. Her email address is email@example.com. The authors would like to thank Roy Bishop of Acadia University for reviewing an earlier draft of this article.
Inside an Atom
Light lets us peek inside the atom -- if we know how to look. According to the simple Bohr model, an atom consists of a nucleus around which electrons buzz in orbits. Each electron orbit represents a discrete energy level; the lowest energy levels are those closest to the nucleus. It takes energy to move up to a higher level.
A photon of light provides the energy that an electron needs to climb up a level. If the photon comes close enough to an atom, it can be absorbed by the atom, pushing the electron up (see diagram below). Depending on how much energy the photon contains, the electron might move up one, two, a few, or many energy levels. If the energy is great enough, the electron may go flying out of the atom altogether.
Atoms are good at absorbing energy, but not so good at holding on to it. Within a few billionths of a second, the electron comes bumping back down to a lower level. Each bump is a step from a higher energy orbit to a lower energy one. At each step, the atom must spit out a photon whose energy equals the energy difference between the two levels.
The key thing is that the atom does not have to release a single photon of light. It can, and often does, release light in a whole series of steps. In this case, the total energy from all the steps must equal the energy of the initially absorbed light. Because there are several outgoing photons, each individual photon is lower in energy -- therefore, longer in wavelength -- than the incoming photon. That's how atoms can turn ultraviolet light into visible light.
Emission from an atom. The Bohr model of the atom gives a rough idea of what happens when an atom absorbs or emits light. The atom looks like a miniature solar system: a nucleus surrounded by electrons in various orbits. In the top case, a photon of short-wavelength light is absorbed by the atom, causing one of its electrons to jump farther away from the nucleus. If this electron falls back down to its original position, it emits a photon of the exact same wavelength. In the bottom case, the electron does not fall to its original position, but rather to an intermediate position. In this case, it emits a photon of lesser energy (longer wavelength). Diagram by Debra A. Fischer.
Dreams of Fields
Opposites attract; like repels. What would pop songs and love sonnets do without the metaphors of magnets? Most of us have played with fridge magnets or compasses; we have seen magnetic poles with the same polarity repel each other and magnetic poles with opposite polarity attract each other. It all depends on the invisible magnetic fields.
Although the concept of a field is abstract, it is easy to envision when you try to push two like magnetic poles together. The magnetic fields penetrate space. They contain energy -- the ability to do work. Slide two magnets with the same polarity toward each other on the surface of a table until they are uncomfortably intimate. If you let go of the magnets, they will scoot away from each other. The energy in the magnetic field is doing work on the magnets.
An analogous situation exists for electric charges. Similar charges repel; opposite charges attract. As with magnetic poles, electric charges are accompanied by electric fields that penetrate space. A negatively charged electron is pulled by the electric field of a positive charge and repelled by the field of a negative charge. Electric fields become even more interesting when they penetrate materials, such as metal wires. There they exert a force that causes the electrons to move through the wire -- the phenomenon of electricity.
But that's not all. Perhaps the most amazing property of electric and magnetic fields is the way they interact with each other. If you take a magnet and plunge it through a loop of wire, an electric field is created. We know an electric field is created because it forces the electrons in the wire to move; we can measure the resulting current. In fact, this is the principle used by electric generators in power stations.
Likewise, moving charges create a magnetic field. To observe this, build a simple circuit with a piece of wire and a battery. Connect one end of the wire to the positive terminal of a 9-volt battery and the other to the negative terminal. Place a compass next to the loop of wire and watch the compass needle move as you connect and disconnect the wire from one of the battery terminals. An important ingredient in both of these experiments is the variation of the fields. Static, unchanging magnetic fields don't spawn electric fields, and steady electric fields don't create magnetic fields.
Once created by a moving magnet or changing electric current, a field can break free of its source. It departs and sails through space like a thought without a thinker. And that is what we call light. Light and other forms of electromagnetic radiation contain both electric and magnetic fields that oscillate in strength. A change in the electric field creates a magnetic field. In return, the oscillating magnetic field creates an electric field. The two fields become entwined in a cyclical dance, each one pushing and then being pulled by the other.
Incredibly, no energy is lost in this process. In a vacuum, an electromagnetic wave would travel forever without losing any energy. It disappears only when it is absorbed by matter -- for instance, a hand that intercepts sunlight and becomes warm. This process of transporting energy is called radiation.
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